Major diameter correcting re-roll thread rolling die

ABSTRACT

A thread profile is first rolled into a cylindrical work piece with a first set of thread dies, wherein the metal free-flows into the grooves of the first set of dies to form a rounded (or other shape) crest. Then the thread is re-rolled in a second set of dies having a reduced groove depth such that the rounded crest is re-formed and flattened. A thread profile may also be formed in a cylindrical work piece using a set of dies having surfaces containing two or more distinct groove shapes. Diagonally cut grooves encountered by the work piece would have a shape substantially identical to a “normal” die groove profile with a depth of D+H. Grooves in contact with the work piece as it exits the die would have a shape with a reduced groove depth of approximately “D.”

RELATED PATENT APPLICATION

This application claims priority to commonly owned U.S. ProvisionalPatent Application Ser. No. 60/648,325; filed Jan. 28, 2005; entitled“Major Diameter Correcting Re-Roll Thread Rolling Die,” by Michael A.Garver; which is hereby incorporated by reference herein for allpurposes.

TECHNICAL FIELD

The present disclosure relates to fastener thread manufacturing dies,more particularly, to a major diameter correcting re-roll thread rollingdie.

BACKGROUND

Referring to FIG. 1, in thread manufacturing today, threads are normallyapplied to externally threaded products such as bolts and machine screwsby a process where the threads are formed by pressing dies 102, ontowhich the shape of the finished thread 104 has been impregnated into acylindrical member 106.

Referring to now FIG. 2, the metal of the cylindrical member 106 isprogressively displaced by the pressing dies 102 as the cylindricalmember 106 is rotated in a direction 208 between opposing faces of thedies 102 a and 102 b having proper shapes for forming the threads on thecylindrical member 106, similar to the action of rolling a pencilbetween one's hands as they are pressed together.

Referring to now FIGS. 3A, 3B and 3C, in order to accomplish this, theshape machined into the dies 102 is a series of parallel grooves 310,cut diagonally across the face of the die 102. Each groove 310 has twoflat sides 312 a and 312 b, each of these sides angled at approximatelythirty degrees, such that the included angle between the two sides 312 aand 312 b of the grooves 310 is approximately sixty degrees. At thebottom of each of the grooves 310 is a flat surface 314, perpendicularto the plane forming the center of the groove 310, and intersecting eachangled side 312 a and 312 b at approximately a thirty degree angle.

Referring now also to FIGS. 4A, 4B and 5, when the metal of thecylindrical member 106, e.g., “work piece,” to be threaded is pressedinto the grooves 310, the cylindrical member 106 metal is displaced soas to fill the grooves 310 in the die 102. Depending on the pressureexerted, the cylindrical member 106, e.g., bolt or machine screw, mayeither partially fill the die groove 310 (represented by 416 of FIG. 4A)or completely fill the die groove (represented by 418 of FIG. 4B). Ifthe pressure is great enough, the resulting shape of the work piecethreads, e.g., 418, may have a shape substantially matching the shape ofthe groove 310 cut in the die 102. This may result in an external threadprofile having two flanks 520 a and 520 b (FIG. 5) and a flat peak or“crest” 522 (FIG. 5).

Referring to FIG. 6, as pressure is reduced, the sharp corners of thedie 102 tend not to fill completely with metal. Rather than being“packed” into a sharp corner, the metal of the work piece 106 may flownaturally in an area where the corners had been. The thread profile maythen appear to have a radius where the sharp corners had been, e.g., 624shown in FIG. 6. As pressure continues to be reduced this natural floweventually forms what appears to be a completely curved surface on thepeak of the thread 418, linking both flanks and forming the “crest” 624of the thread 418. Further reducing pressure may result in a curvedsurface degrading into a “ragged” appearing crest (726 of FIG. 7), theneventually, with lower pressure and because of lack of metal, the flanks520 a and 520 b may themselves begin to degrade. Typically, standardexternally threaded fasteners may tend to have some variant of theseshapes on the crest of the threads.

In fastener manufacture, production processes for both dies and threadshave significant inherent variations. Because of these variations, it isvery uncommon in the thread rolling process to completely fill thecorners of the die groove as first discussed above and shown in FIG. 4B.What is more common, are various degrees of corner fill depending uponthe tolerances of the cylindrical work piece 106 and dies 102, and theamount of pressure applied during actual production thereof.

Due to this phenomenon, die manufacturing specifications are such thatit is assumed the that the normal result of the rolling process will notbe a thread profile with sharp corners, e.g., crest 522 of FIG. 5, butsome variant of the crest shape discussed above and shown in FIGS. 6 and7. The most common shape seen in commerce today is the “fully radiused”crest 624 shown in FIG. 6.

In order to allow this condition, commonly known as “underfill” of thedie, the depth of the flat bottom of the groove in the die is normallymade deeper than expected. Thus, there is “extra” room in the die, sothat any metal forming the curved crest seen on the resulting threadprofile 828 of the work piece shown in FIG. 8, exceeds the metalrequired to obtain the minimum flank height of the thread profilerepresented by 830 of FIG. 8. This, therefore, insures that the threadflanks 520 a and 520 b are high enough to meet minimum thread strengthcharacteristics while allowing for excess metal flow.

Thread rolling die specifications, therefore, are set up not to matchthe desired shape, but to exceed the specified envelope of the minimumheight of the actual thread profile, as designated in ISO and otherthread standard specifications, incorporated by reference herein for allpurposes. Minimum flank height and thread strength required by thespecification thereby may be maintained.

Referring to now to FIG. 9, depicted is a comparison of an M8×1.25 6 gexternal thread tolerance envelope (on the right) and the accompanyingdie dimension envelope (on the left). Note that the die envelope showsthe desired die tolerances 932, shown solid black, along with thepermissible die movement allowed by thread pitch diameter tolerances934. The thread envelop is shown with its entire permissible major,minor and pitch diameter variations shaded and represented as 936. Notethat the peak of the die specification, when adjusted to maximum pitchdiameter may significantly exceed the maximum permissible major diameterof the thread.

The threads formed thusly, therefore tend to have a major diameter (forpracticality, measured across the rounded crests of the threads) that islarger than expected for a thread of that pitch diameter. These threadsalso have smaller flank height than expected for such a major diameter.It, in fact, means that external threads produced with dies made topresent day die specifications can, within the permissible range ofpitch diameters, exceed the permissible major diameter of the thread,while having flank heights that are within specification. Thispotentially leads to internal/external thread interference in use. Thisoften leads manufacturers to make their fasteners to the low end of thepitch diameter range, in order that the measured major diameter does notexceed the maximum specification. It also means that threads produceddeliberately to the low end of the pitch range to allow for thickplating on the surface of the thread, potentially have flank heightbelow that required for minimum strength requirements.

SUMMARY

Therefore, there is a need for a method of and apparatus formanufacturing threads that do not ultimately result in an undesirablerounded crest, thus allowing thread major diameters to be made to thespecified diameter, with sharp flank-crest interfaces and no excessmetal beyond said diameter, while not significantly affecting thecurrent thread manufacturing process. Such a method of and apparatus formanufacturing threads would guarantee intended thread strengthcharacteristics across the entire range of possible thread and diedimensional envelopes, thus allowing threads to be deliberatelymanufactured to the lower end of the pitch diameter range to permitpainting and/or plating of the fastener and also allow interfaces withpainted internal threads.

According to a specific example embodiment of the present disclosure, anapparatus for rolling threads on a cylindrical work piece and havingmajor diameter thread crest correction may comprise a first die set forrolling threads on a cylindrical work piece, wherein the first die sethas thread forming grooves at a first depth; and a second die set forre-rolling the threads on the cylindrical work piece, wherein the seconddie set has thread forming grooves at a second depth, wherein the seconddepth is less than the first depth. The second die set thread forminggrooves may reshape a crest of the threads on the cylindrical work pieceto a desired height. The second die set thread forming grooves mayreshape a crest of the threads on the cylindrical work piece to a desireshape. The second die set thread forming grooves may reshape a crest ofthe threads on the cylindrical work piece to a substantially flat peak.

According to another specific example embodiment of the presentdisclosure, an apparatus for rolling threads on a cylindrical work pieceand having major diameter thread crest correction may comprise a die setfor rolling threads on a cylindrical work piece, wherein the die set hasa first portion with thread forming grooves at a first depth, and asecond portion with thread forming grooves at a second depth, whereinthe second depth is less than the first depth; wherein during threadrolling on the cylindrical work piece in the first portion of the dieset the thread crests are at substantially the first depth, and duringthread rolling on the cylindrical work piece in the second portion ofthe die set the thread crests are at substantially the second depth. Thesecond portion with thread forming grooves at the second depth mayreshape a crest of the threads on the cylindrical work piece to adesired height. The second portion with thread forming grooves at thesecond depth may reshape a crest of the threads on the cylindrical workpiece to a desired shape. The second portion with thread forming groovesat the second depth may reshape a crest of the threads on thecylindrical work piece to a substantially flat peak.

According to yet another specific example embodiment of the presentdisclosure, an apparatus for rolling threads on a cylindrical work pieceand having major diameter thread crest correction may comprise a die setfor rolling threads on a cylindrical work piece, wherein the die setcomprises a plurality of portions having progressively smaller groovedepths, whereby during thread rolling the cylindrical work piece hasthread crest heights at substantially the groove depths of each of theplurality of portions of the die set and at the end of thread rollinghas a thread crest height at substantially a groove depth of a last oneof the plurality of portions of the die set. The thread forming groovesof the last one of the plurality of portions of the die set are at adepth and shape whereby the crest of the threads on the cylindrical workpiece are reshaped to a desired height. The thread forming grooves ofthe last one of the plurality of portions of the die set are at a depthand shape whereby the crest of the threads on the cylindrical work pieceare reshaped to a desired shape. The thread forming grooves of the lastone of the plurality of portions of the die set are at a depth and shapewhereby the crest of the threads on the cylindrical work piece arereshaped to a substantially flat peak.

According to still another specific example embodiment of the presentdisclosure, a method for rolling threads on a cylindrical work piece andfor correcting a major diameter thread crest may comprise the steps ofrolling threads on a cylindrical work piece with a first die set,wherein the first die set has thread forming grooves at a first depth;and re-rolling the threads on the cylindrical work piece with a seconddie set, wherein the second die set has thread forming grooves at asecond depth, wherein the second depth is less than the first depth. Thestep of re-rolling the threads on the cylindrical work piece with thesecond die set may include the step of reshaping a crest of the threadson the cylindrical work piece to a desired height. The step ofre-rolling the threads on the cylindrical work piece with the second dieset may include the step of reshaping a crest of the threads on thecylindrical work piece to a desired shape. The step of re-rolling thethreads on the cylindrical work piece with the second die set mayinclude the step of reshaping a crest of the threads on the cylindricalwork piece to a substantially flat peak.

According to another specific example embodiment of the presentdisclosure, a method for rolling threads on a cylindrical work piece andfor correcting a major diameter thread crest may comprise rollingthreads on a cylindrical work piece with a die set, wherein the die setcomprises a first portion with thread forming grooves at a first depth,and a second portion with thread forming grooves at a second depth,wherein the second depth is less than the first depth; wherein duringthe step of rolling threads in the first portion of the die set thecylindrical work piece has thread crests at substantially the firstdepth and during the step of rolling threads in the second portion ofthe die set the cylindrical work piece has thread crests atsubstantially the second depth. The step of rolling threads in thesecond portion of the die set may include the step of reshaping a crestof the threads on the cylindrical work piece to a desired height. Thestep of rolling threads in the second portion of the die set may includethe step of reshaping a crest of the threads on the cylindrical workpiece to a desired shape. The step of rolling threads in the secondportion of the die set may include the step of reshaping a crest of thethreads on the cylindrical work piece to a substantially flat peak.

According to yet another specific example embodiment of the presentdisclosure, a method for rolling threads on a cylindrical work piece andfor correcting major diameter thread crests may comprise the steps ofrolling threads on a cylindrical work piece with a die set, wherein thedie set comprises a plurality of portions with thread forming grooves atprogressively smaller depths; wherein during the step of rolling threadson the cylindrical work piece the threads are formed to a desired shape.The step of rolling threads may include the step of reshaping a crest ofthe threads to a desired height. The step of rolling threads may includethe step of reshaping a crest of the threads to a substantially flatpeak.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present disclosure thereof may beacquired by referring to the following description taken in conjunctionwith the accompanying drawings wherein:

FIG. 1 is a schematic cross-section view of pressing dies impregnatingfinished threads into a cylindrical member, according to priortechnology;

FIG. 2 is schematic elevational view of pressing dies impregnatingfinished threads into a cylindrical member, according to priortechnology;

FIG. 3A is a front view of a die face showing the machined series ofparallel groves;

FIG. 3B is a cross-section view A-A of the groves in the die face shownin FIG. 3A;

FIG. 3C is a magnified cross-section view B of a portion of the grovesin the die face shown in FIG. 3B;

FIG. 4A is a cross-section view of a die face grove and thread formingin the cylindrical member when low pressure is exerted on the workpiece;

FIG. 4B is a cross-section view of a die face grove and thread formingin the cylindrical member when high pressure is exerted on the workpiece;

FIG. 5 is a cross-section view of a thread formed when high pressure isexerted on the work piece;

FIG. 6 is a cross-section view of a thread formed when reduced pressureis exerted on the work piece;

FIG. 7 is a cross-section view of a thread formed when even lesspressure is exerted on the work piece;

FIG. 8 is a cross-section view of an curved crest formed in excess on athread in relation to a desired thread profile;

FIG. 9 shows a die profile envelope in comparison to a thread profileenvelope;

FIG. 10 shows a two step process for forming threads in a work piece,according to a specific example embodiment of the present disclosure;

FIG. 11 shows a single step process for forming threads in a work pieceusing a set of dies having surfaces containing two or more distinctgroove shapes, according to another specific example embodiment of thepresent disclosure; and

FIG. 12 shows a comparison of a desired normal crest height and formingcrest heights necessary to equalize the total area so as to obtainsubstantially the desired normal crest height.

While the present disclosure is susceptible to various modifications andalternative forms, specific example embodiments thereof have been shownin the drawings and are herein described in detail. It should beunderstood, however, that the description herein of specific exampleembodiments is not intended to limit the disclosure to the particularforms disclosed herein, but on the contrary, this disclosure is to coverall modifications and equivalents as defined by the appended claims.

DETAILED DESCRIPTION

Referring now to the drawings, the details of example embodiments areschematically illustrated. Like elements in the drawings will berepresented by like numbers, and similar elements will be represented bylike numbers with a different lower case letter suffix.

Referring to FIG. 10, depicted is a two step process for forming threadsin a work piece, according to a specific example embodiment of thepresent disclosure. A thread profile is first rolled into a cylindricalmember 106 work piece with a first set of thread dies 1038 as is donetoday (as shown in the top section details of FIG. 10), allowing themetal to free-flow into the corners of the grooves of the first set ofdies 1038, resulting in an under-filled, rounded, or other shape crest.Then the thread is re-rolled in a second set of dies 1040 having areduced groove depth (as shown in the bottom section details of FIG.10), such that the rounded (or other shape) crest is re-formed andflattened, thus forcing its height down to a desired height and into thecorners of the grooves of the second set of thread dies 1040 (as shownin the bottom section details of FIG. 10).

Referring to FIG. 11, depicted is a single step process for formingthreads in a work piece using a set of dies having surfaces containingtwo or more distinct groove shapes, according to another specificexample embodiment of the present disclosure. Diagonally cut groovesencountered by the work piece cylinder, as it enters the threading dieset at the feed end 1144 would have a shape substantially identical tothe “normal” die groove profile, e.g., as shown in FIG. 3C, with a depthof D+H (FIG. 10), or such other shape as may be efficiently re-rolled,per the process described below. Grooves in contact with the work pieceas it exits the die at the exit end 1146, would have a shape similar tothe shape shown in FIG. 3C, but with a reduced groove depth ofapproximately “D” (FIG. 10).

Depth “D” of FIG. 10 is the nominal design height of the thread flankprofile intended to be formed, shown as 1242 in FIG. 12. Depth “H” maybe defined by computing the height necessary to equalize the total areaof the shaded sections 1244 and 1246 (FIG. 12), on either side of 1242.“H” is then considered an approximate basis for the reduction in diegroove depth, required to re-roll the crest profile to achieve a finalflank height of “H” on the work piece. The “H” dimension may be variedslightly to obtain the best results in actual practice, such that thereduction produces the desired thread shape and dimensionalcharacteristics.

According to the teachings of this disclosure, such crest shape changemay be accomplished by either altering the shape of each groove as itprogresses toward the exit end 1146 of the die 1142, or, by maintaininga constant shape in each groove, but, giving grooves at the entrance end1144 of the die a deeper shape and those at the exit end 1146 of the die1142 a shallower shape. The later of these may require die lengthslonger than those normally associated with standard fastenermanufacturing.

It is contemplated and within the scope of the teachings of thisdisclosure that the change in depth of the die groove may beaccomplished in one or more steps such that the revolving work piece mayencounter different depth and/or shape grooves every time it touches adie, as it progresses through the die set, or groove depth may bechanged in one step. Such a die might provide one step, intermediatestepped grooves, or a constantly changing groove depth, so long as thedesired change in crest height and shape ultimately results.

Intermediate groove shapes may also be present throughout the die, solong as the desired change in crest height and shape ultimately results.Inadvertent and/or deliberately created crest shape variations areanticipated during the rolling process, as the work piece progressesfrom the beginning to end of the die. Grooves not in contact with thework piece at the entrance or exit ends of the die, may have either ofthe shapes in question, or both, or any other shape which allows for there-rolling to be accomplished. Re-rolling as described herein may beaccomplished by one or both of the dies in the rolling process, and thateither of the dies may be a “normal” die, or itself be a specialize diefor another purpose. Each die may have a different progression ofdifferent groove depths, as discussed above, or may differ from theopposing die in any of the variants discussed hereinabove. There-rolling process may be accomplished in a variety of other styles ofthread rolling dies, such as, but not limited to, cylindrical dies,and/or segmented dies. Either or both of the such dies may also becombined with other thread shapes, such as, but not limited toanti-cross-thread type dies, which may or may not be re-rolled by asimilar process.

While embodiments of this disclosure have been depicted, described, andare defined by reference to example embodiments of the disclosure, suchreferences do not imply a limitation on the disclosure, and no suchlimitation is to be inferred. The subject matter disclosed is capable ofconsiderable modification, alteration, and equivalents in form andfunction, as will occur to those ordinarily skilled in the pertinent artand having the benefit of this disclosure. The depicted and describedembodiments of this disclosure are examples only, and are not exhaustiveof the scope of the disclosure.

1. An apparatus for rolling threads on a cylindrical work piece andhaving major diameter thread crest correction, comprising: a first dieset for rolling threads on a cylindrical work piece, wherein the firstdie set has thread forming grooves at a first depth; and a second dieset for re-rolling the threads on the cylindrical work piece, whereinthe second die set has thread forming grooves at a second depth, whereinthe second depth is less than the first depth.
 2. The apparatusaccording to claim 1, wherein the second die set thread forming groovesreshapes a crest of the threads on the cylindrical work piece to adesired height.
 3. The apparatus according to claim 1, wherein thesecond die set thread forming grooves reshapes a crest of the threads onthe cylindrical work piece to a desire shape.
 4. The apparatus accordingto claim 1, wherein the second die set thread forming grooves reshapes acrest of the threads on the cylindrical work piece to a substantiallyflat peak.
 5. An apparatus for rolling threads on a cylindrical workpiece and having major diameter thread crest correction, comprising: adie set for rolling threads on a cylindrical work piece, wherein the dieset has a first portion with thread forming grooves at a first depth,and a second portion with thread forming grooves at a second depth,wherein the second depth is less than the first depth; wherein duringthread rolling on the cylindrical work piece in the first portion of thedie set the thread crests are at substantially the first depth, andduring thread rolling on the cylindrical work piece in the secondportion of the die set the thread crests are at substantially the seconddepth.
 6. The apparatus according to claim 5, wherein the second portionwith thread forming grooves at the second depth reshapes a crest of thethreads on the cylindrical work piece to a desired height.
 7. Theapparatus according to claim 5, wherein the second portion with threadforming grooves at the second depth reshapes a crest of the threads onthe cylindrical work piece to a desired shape.
 8. The apparatusaccording to claim 5, wherein the second portion with thread forminggrooves at the second depth reshapes a crest of the threads on thecylindrical work piece to a substantially flat peak.
 9. An apparatus forrolling threads on a cylindrical work piece and having major diameterthread crest correction, comprising: a die set for rolling threads on acylindrical work piece, wherein the die set comprises a plurality ofportions having progressively smaller groove depths, whereby duringthread rolling the cylindrical work piece has thread crest heights atsubstantially the groove depths of each of the plurality of portions ofthe die set and at the end of thread rolling has a thread crest heightat substantially a groove depth of a last one of the plurality ofportions of the die set.
 10. The apparatus according to claim 9, whereinthe thread forming grooves of the last one of the plurality of portionsof the die set are at a depth and shape whereby the crest of the threadson the cylindrical work piece are reshaped to a desired height.
 11. Theapparatus according to claim 9, wherein the thread forming grooves ofthe last one of the plurality of portions of the die set are at a depthand shape whereby the crest of the threads on the cylindrical work pieceare reshaped to a desired shape.
 12. The apparatus according to claim 9,wherein the thread forming grooves of the last one of the plurality ofportions of the die set are at a depth and shape whereby the crest ofthe threads on the cylindrical work piece are reshaped to asubstantially flat peak.
 13. A method for rolling threads on acylindrical work piece and for correcting a major diameter thread crest,said method comprising the steps of: rolling threads on a cylindricalwork piece with a first die set, wherein the first die set has threadforming grooves at a first depth; and re-rolling the threads on thecylindrical work piece with a second die set, wherein the second die sethas thread forming grooves at a second depth, wherein the second depthis less than the first depth.
 14. The method according to claim 13,wherein the step of re-rolling the threads on the cylindrical work piecewith the second die set includes the step of reshaping a crest of thethreads on the cylindrical work piece to a desired height.
 15. Themethod according to claim 13, wherein the step of re-rolling the threadson the cylindrical work piece with the second die set includes the stepof reshaping a crest of the threads on the cylindrical work piece to adesired shape.
 16. The method according to claim 13, wherein the step ofre-rolling the threads on the cylindrical work piece with the second dieset includes the step of reshaping a crest of the threads on thecylindrical work piece to a substantially flat peak.
 17. A method forrolling threads on a cylindrical work piece and for correcting a majordiameter thread crest, said method comprising the steps of: rollingthreads on a cylindrical work piece with a die set, wherein the die setcomprises a first portion with thread forming grooves at a first depth,and a second portion with thread forming grooves at a second depth,wherein the second depth is less than the first depth; wherein duringthe step of rolling threads in the first portion of the die set thecylindrical work piece has thread crests at substantially the firstdepth and during the step of rolling threads in the second portion ofthe die set the cylindrical work piece has thread crests atsubstantially the second depth.
 18. The method according to claim 17,wherein the step of rolling threads in the second portion of the die setincludes the step of reshaping a crest of the threads on the cylindricalwork piece to a desired height.
 19. The method according to claim 17,wherein the step of rolling threads in the second portion of the die setincludes the step of reshaping a crest of the threads on the cylindricalwork piece to a desired shape.
 20. The method according to claim 17,wherein the step of rolling threads in the second portion of the die setincludes the step of reshaping a crest of the threads on the cylindricalwork piece to a substantially flat peak.
 21. A method for rollingthreads on a cylindrical work piece and for correcting major diameterthread crests, said method comprising the steps of: rolling threads on acylindrical work piece with a die set, wherein the die set comprises aplurality of portions with thread forming grooves at progressivelysmaller depths; wherein during the step of rolling threads on thecylindrical work piece the threads are formed to a desired shape. 22.The method according to claim 21, wherein the step of rolling threadsincludes the step of reshaping a crest of the threads to a desiredheight.
 23. The method according to claim 21, wherein the step ofrolling threads includes the step of reshaping a crest of the threads toa substantially flat peak.